Faster, more conclusive hevm test on spark-psm: invariant state merging, diamond merges, lemmas, memory fixes - #1248

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, nix-community/cache-nix-action@135667ec418502fa5a3598af6fb9eb733888ce6a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/